The present invention generally relates to a detonator as well as to an initiating element and an associated method.
Detonators are used either as an explosive per se or to detonate other explosives.
In a typical embodiment, a detonator comprises a shell having a closed end against which a base charge is packed or pressed. In the other end of the shell, an igniting means, such as a pyrotechnical fuse, a NONEL(copyright) tube or an electric fuse head, is arranged. Between the igniting means and the base charge, an initiating charge is arranged, which can be ignited by the igniting means. The combustion of the initiating charge initiates the detonation of the base charge.
Explosives are roughly divided into primary explosives and secondary explosives. The primary explosives are characterised in that they are able to develop full detonation out of being heated when present in small quantities in a free state i.e. when unconfined. On the other hand, the secondary explosives need to be confined and require greater quantities or heavy mechanical impact to develop detonation. For security reasons, use of primary explosives is often avoided, and the present invention only relates to detonators which are free from primary explosives. As examples of secondary explosives mention can be made of PETN (pentaerythritoltetranitrate), HMX (cyclotetramethylenetetranitramine), RDX (phlegmatised hexgen, cyclotrimethylenetrinitramine), TNT (trinitrotoluene), Tetryl (trinitrophenylmethylnitramine) and mixtures of one or more of these.
There is a quadratic relation between the detonation speed of an explosive and the shock wave energy which develops at the detonation. In order to obtain the greatest possible explosive effect, a high detonation speed must therefore be provided. This is the case in particular with detonators which are used for detonation of other explosives, since the detonators generally contain only a small amount of secondary explosive, which should thus detonate at the highest possible speed to achieve a maximum explosive effect.
The detonation speed of an explosive increases as the density of the explosive increases. The detonation speed of phlegmatised hexogen (RDX) is, for instance, 8.7 km/s at the density 1.8 g/cm3, whereas it is only 7.6 km/s at the density 1.5 g/cm3, which corresponds to a reduction of the shock wave energy by almost 30%.
Detonators according to prior-art technique are provided with a base charge which is usually pressed to a density of about 1.5-1.55 g/cm3. Even if higher density is desirable, this has not been feasible in practice.
The main object of the invention is to provide a detonator which, given a certain amount of explosive in the base charge, yields higher shock wave energy than allowed by prior-art technique.
A more concrete object of the invention is to provide further increased density in a base charge pressed into a detonator, thereby to provide an increased detonation speed, and thus enhanced explosive effect, of the detonation charge.
Another object of the invention is to provide an initiating element for use in a detonator, said initiating element allowing further increased density to be imparted to a base charge pressed into the detonator, said density being maintained until the base charge is caused to detonate.
These objects are achieved by means of a method and a detonator or an initiating element according to the appended claims.
Thus the invention is based on the knowledge that a detonator can exhibit enhanced explosive effect given a certain amount of explosive in the base charge if increased density has been imparted to this base charge substantially at the moment of detonation. If the base charge is compressed to such a degree that at least some part thereof attains a substantially crystalline state just before, and during, the detonation, a significantly enhanced explosive effect is provided.
According to one aspect of the invention, use is made of the pressure which arises in the combustion of an initiating charge to further increase the density of an already compressed base charge and to maintain the high density until the bass charge is caused to detonate, resulting in an increased detonation speed and thus enhanced explosive effect. Preferably, such high density of the base charge is provided that the latter, at least partially, attains a substantially crystalline state.
According to another aspect of the invention, the combustion gases from an initiating charge are used to heat until ignition and to compress a loosely packed, or unconfined, secondary explosive whose energy is thus increased, which finally results in detonation of this secondary explosive which thus causes a base charge which is compressed to increased density to detonate.
According to yet another aspect of the invention, an initiating element is provided for use in a detonator to cause a compressed base charge which is arranged in the detonator to detonate.
The initiating element according to the invention comprises a compression means which is arranged to be acted upon by combustion gases, which develop in the combustion of an initiating charge, in order to further compress the base charge.
According to the invention, an initiating element is also provided, which allows hot combustion gases from the combustion of the initiating charge to pass into a chamber which is arranged in the initiating element and which is adjacent to a base charge arranged outside the initiating element. In the chamber, a loosely pressed or unconfined secondary explosive is preferably arranged, which is intended to be heated until ignition by the entering combustion gases, whereby said base charge is finally caused to detonate.
The invention also relates to an initiating element which uses the above-mentioned combustion gases to heat and compress the loosely pressed secondary explosive to cause the same to detonate, at the same time as the compressed base charge is exposed to a force, which originates from the burning initiating charge, which force further increases the density of the base charge, at least some part of the base charge attaining a substantially crystalline state. Preferably, the loosely pressed secondary explosive is already heated until ignition when the compression thereof begins to take effect.
According to the invention, a base charge in the detonator, which is compressed when manufacturing a detonator, is thus caused to detonate with the aid of an initiating charge by means of a method in which the pressure which develops in the combustion of the initiating charge is used to further compress the base charge before the detonation thereof.
According to a preferred embodiment of the invention, the initiating element comprises a secondary explosive which is arranged to cause detonation of the base charge in a detonator.
In a particularly preferred embodiment of an initiating element according to the invention, the secondary explosive of the initiating element causes detonation of the base charge by said secondary explosive being heated until ignition and compressed by means of combustion gases which develop in the combustion of an initiating charge arranged in the initiating element.
One embodiment of a detonator according to the invention may thus comprise an initiating element having a chamber which is connected with a base charge, said chamber containing a comparatively loosely pressed or unconfined secondary explosive. During an initiation phase, i.e. in the combustion of an initiating charge, the volume of said chamber is reduced, resulting in a pressure rise in said chamber. At the same time, the combustion of the initiating charge causes further compression of the base charge which thus attains a substantially crystalline or at least very compressed state. The ignition of the base charge is provided by the burning gases in the initiating charge passing into said chamber, whereby the explosive in this chamber is heated until ignition. When the explosive in the chamber has been heated until ignition, the pressure, and thus the energy, in the chamber is increased so that this explosive finally attains detonation, whereby the base charge in caused to detonate.
In preferred embodiments, the pressure rise in said chamber is provided by a positive pressure which is caused by the initiating charge pushing a movably arranged piston into the chamber, so that the volume thereof is reduced. Preferably, the thickness of the piston is greater than 0.15 mm and smaller than 1.0 mm.
The diameter of the above-mentioned chamber is preferably greater than the critical detonation diameter of the explosive which is intended to be placed in the chamber. The critical detonation diameter for PETN (pentaerythritoltetranitrate) is, for instance, about 1 mm. Furthermore, it has been found that the length of the chamber (its axial extension) is advantageously greater than its diameter, but smaller than about ten times its diameter.
Moreover, in preferred embodiments use is made of a suitably piston-shaped compression means to provide said further compression of the base charge, the above-mentioned chamber being arranged as a preferably axial duct in the compression means. It has been found that the diameter of the compression means is advantageously at least 1.1 times greater than the diameter of such a duct. More preferably, it is at least 1.5 times greater and most preferably about two times greater than the diameter of the duct.
The present invention allows manufacture of initiating elements having a total length of 9-10 mm, which is comparable with the primary explosive charge in detonators according to prior-art technique, in which the length of the column of primary explosive in the initiating charge is typically about 6-7 mm.